1. Field of the Invention
The present invention relates generally to a switching regulator and more specifically to a switching regulator of a type wherein a switching control arrangement includes an optical coupler. The present invention features the provision of an improved switching control arrangement which facilitates circuit design and which enables a stable dc (direct current) voltage to be derived.
2. Description of the Prior Art
In order to obtain a dc power output without an undesirable increase in the number of electronic components, it is a known practice to implement dc-to-dc conversion of a rectified but unregulated ac (alternate current) line voltage through the use of a switching regulator. This kind of power supply circuitry is currently employed in a variety of electronic equipment.
Before describing in detail the present invention a known switching regulator will be discussed with reference to FIG. 1.
Input terminals 11 and 12 of a switching regulator 10 are supplied with a rectified but unregulated ac line voltage (100 V by way of example). A capacitor 14, coupled across the input terminals 11, 12, functions as a capacitor filter. A transformer 16 is provided with a primary winding N1 and a secondary winding N2. The winding N1 and a switching transistor 18 are coupled in series across the input terminals 11 and 12. The transistor 18 is selectively rendered conductive (viz., switched on and off) by a train of pulses applied thereto from a switching transistor controller 20.
An ac voltage developed across the secondary winding N2 is rectified and smoothed at a rectifier/smoothing circuit 22. The output voltage Va of the circuit 22 is applied to output terminals 24, 26 and also applied to one input terminal 28a of an error amplifier 28 which forms part of a switching control arrangement. The amplifier 28 is further supplied with a reference voltage Vref via the other input terminal 28b, and is arranged to generate an error voltage Vb which corresponds to an amplified difference voltage between the two input voltages Va and Vref. The output voltage Vb (analog) is applied to an optical coupler 30 which generates an analog output voltage Vc proportional to the error voltage Vb applied thereto. The optical coupler 30 electrically isolates the input and output of the switching control arrangement.
A comparator 32 receives the analog voltage Vc from the optical coupler 30 and also receives a reference voltage Vd from a reference signal generator 34. The reference voltage Vd takes the form of a triangular or saw tooth wave by way of example. An output voltage Ve of the comparator 32, in the form of consecutive pulses, is applied to the switching transistor controller 20 which controls the transistor 18 by varying the duty cycle thereof. The reference signal or voltage Vd has a frequency which is identical with the switching frequency of the transistor 18.
With the arrangement as described above, in the event that the output voltage Va exceeds the reference voltage Vref, the output Vb of the error amplifier 28 rises accordingly. This allows the optical coupler 30 to generate its output voltage Vc higher than in the case of a stable state of the regulator 10, thereby to reducing the width of each of the pulses derived from the comparator 32. Consequently, the "on" time duration of the transistor 18 is shortened which causes the dc output voltage Va to become lower. Thus, the dc output voltage Va is lowered or adjusted toward the predetermined value (Vref). On the contrary, in the case that the output dc voltage Va is lowered below the reference voltage Vref, as will be understood from the above description, the output dc voltage Va is controlled to the predetermined value (Vref) by the above-mentioned switching technique.
As previously mentioned, the dc voltage applied to the input terminals 11 and 12, is a rectified but unregulated ac line voltage and hence it is inherently susceptible to external influences and/or noise. That is to say, the output of the optical coupler tends to be susceptible to ambient or environmental temperature.
Further, as it is virtually impossible to obtain any two optical couplers with exactly the same frequency verses gain and phase characteristics, in the event that the optical coupler in a circuit fails and requires replacement, a problem arises in that pains are required to restore the circuit to a suitable operating condition. In some cases the regulated dc output may become unstable or in the worst case, the amplifier may undergo oscillation.
Thus, as will be appreciated, in addition to the problems encountered during replacement, the optical coupler inherently introduces the need for an abnormally high degree of care to be exercised when designing and assembling a circuit of the nature in question in order to obtain a stable dc output.